Heart valve stent

ABSTRACT

An anatomically configured stent for a tanned, expanded natural tissue heart valve and a valve prosthesis including the stent are disclosed. The stent comprises a frame whose interior configuration is the anatomical negative of the exterior configuration of a tanned, expanded, natural tissue valve and a fabric cover covering at least the exterior of the frame, the cover having a bead along its perimeter that provides a site for attachment of the valve to the stent.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of application Ser. No.536,300, filed 20 Jan. 1975, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a stent for tanned, natural tissue heartvalves and to a heart valve prosthesis including the stent.

2. Description of the Prior Art

There are basically two types of heart valve prostheses: mechanicalvalve prostheses and natural tissue valve prostheses. Recent experienceindicates the tissue type may be superior to the mechanical type asregards similarity to natural flow characteristics, thrombogenicity andincidence of catastrophic in vivo dysfunction.

It is known that the tensile properties and antigenic reaction ofnatural tissue valves may be improved if they are tanned before beingimplanted. Glutaraldehyde has been used as a tanning agent. (Yarbroughet al: Structural alterations in tissue cardiac valves implanted inpatients and in calves. J Thoracic and Cardiovascular Surgery 65, March1973, pp 364-74.)

Heart valve prostheses desirably include a stent or support in which thevalve is held in place. The stents normally comprise a frame coveredwith a cloth or sponge sleeve. Variously configured frames and framematerials have been used in an attempt to provide a stent that does notcontribute to in vivo valve dysfunction. For instance U.S. Pat. No.3,570,014 discloses a stent made of an annular bendable tubularframework, portions of which are covered with fabric to provide anattachment site for the valve. The frame is bent to fit the valve.Another tubular framework stent is disclosed in U.S. 3,755,823. Thatstent is made of spaced annular tube members that carry threedeflectable posts. The framework is covered with a fabric sleeve.Carpentier (Human Organ Support and Replacement: Transplantation andArtificial Prostheses. J Hardy, ed, Springfield, I11, Charles C Thomas,1971, pp 332-62) reports stented valves in which an attempt was made tomake the stent frames anatomical by dimensioning them according tomeasured inside valve dimensions. Even so, these prior art stents wereonly partial or poor facsimilies of a truly anatomically configuredstent.

SUMMARY OF THE INVENTION

The invention is an anatomically configured stent for a natural, tissueheart valve that has been tanned and expanded simultaneously withglutaraldehyde at a positive pressure and a heart valve prosthesiscomprising such a valve affixed within the stent. The stent comprises: aplastic frame the inner configuration of which is substantiallyidentical to the exterior configuration of the tanned, expanded valve;and a fabric cover covering at least the exterior surface of the frame,said cover having means along its perimeter providing a site forattachment of the valve to the stent.

The basic features that distinguish the invention stent from prior artstents are: (1) it is designed to support a tissue valve which has beentanned and expanded as described above; and (2) its inner surface isanatomically configured to conform nearly exactly to the exteriorsurface of the valve. An ancillary distinguishing feature of anembodiment of the invention stent in which the fabric cover extends overonly the exterior surface of the stent is that the cover is heatlaminated directly to the stent frame thereby providing a strong,effective union between cover and frame.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is an enlarged, elevational view of an embodiment of the heartvalve prosthesis of this invention;

FIG. 2 is a top plan view of the heart valve prosthesis of FIG. 1;

FIG. 3 is a top plan view of the stent, without fabric covering, of theheart valve prosthesis of FIG. 1;

FIG. 4 is an exploded view showing the four basic elements of the heartvalve prosthesis of FIG. 1;

FIG. 5 is a developmental layout view of the frame of FIG. 4approximately two times actual scale;

FIG. 6A is a sectional view taken along line 6A-6A of FIG. 5;

FIG. 6B is a sectional view taken along line 6B-6B of FIG. 5;

FIG. 6C is a sectional view taken along line 6C-6C of FIG. 5;

FIG. 6D is a sectional view taken along line 6D-6D of FIG. 5;

FIG. 6E is a sectional view taken along line 6E-6E of FIG. 5;

FIG. 6F is a sectional view taken along line 6F-6F of FIG. 5;

FIG. 7 is an enlarged vertical sectional view through an apical portionof the stent of the heart valve prosthesis of FIG. 1;

FIG. 8 is a bottom plan view of the heart valve prosthesis of FIG. 1;

FIG. 9 is an enlarged, elevational view of a second embodiment of theheart valve stent of this invention;

FIG. 10 is an enlarged, elevational view of the frame of the stent ofFIG. 9;

FIG. 11 is an enlarged vertical sectional view through an apical portionof the stent of FIG. 9; and

FIG. 12 is a developmental layout view of the frame of the stent of FIG.9 approximately two times actual scale.

DETAILED DESCRIPTION OF THE INVENTION

The valves used in the prostheses of the invention are tanned, expandednatural tissue cardiac valves. Homograft or heterograft valves may beused. For implantation in humans aortic porcine valves are preferredbecause they are very similar to human valves and easy to procure. Also,their uniformity (and reproducibility) may be controlled through controlof the donor population. A series of porcine valves taken from donors atages (approximate) 3 weeks, 3 months, 6 months, 1 year, and 2 yearsprovides an excellent selection of valve sizes for use in human heartvalve prostheses. These valves will range in size from about 20 to about35 mm outside diameter.

In order to minimize the deleterious effect of autolysis andbacterial/enzymatic action of the valve tissue, the valves should beprocured fresh and kept in chilled saline pending tanning. Forconvenience the entire aortic root is taken and tanned, and the valve isdissected from the root after tanning. It is desirable that the valvesbe tanned within about one-half day of their procurement; otherwise theymay become flaccid and lose their normal firm texture. The tanningimproves the tensile (strength) properties of the valves and improvestheir resistance to reactivity in vivo to body tissue and fluids. It isessential that the tanning be carried out so as to substantiallymaintain the natural configuration of the valve despite the tanning. Itwas found that this could be accomplished by using a 0.5% to 10%glutaraldehyde solution as the tanning agent and by keeping the valveexpanded during the tanning. Such expansion may be effected by applyinga positive pressure of about 20 mm Hg to about 100 mm Hg, preferablyabout 50 mm Hg to the valves. Pressure may be exerted on the valves by ahydrostatic column attached to the valve cuff, the valves beingeffectively sealed to maintain the pressure by closure of the valvecusps and by ligation of arteries. The time over which the valve shouldbe kept in contact with the glutaraldehyde solution to achieve thedesired degree of tanning varies with the glutaraldehyde concentration.At the above stated concentrations the valves should be kept in thesolution for about ten minutes to one hour. For instance, treatment with5% glutaraldehyde solution for approximately ten minutes at ambienttemperature provided valves with good configuration maintenance.

After tanning the valves may be stored in a 0.1% to 0.8% glutaraldehydesolution to prevent them from drying out. At such concentrationsglutaraldehyde also has sufficient cidal activity to keep the tannedvalves sterile. Caution should be taken against storing the valves inmore concentrated solutions since the same may cause the valve cusps tostiffen and thus be susceptible to in vivo failure due to stressfracture. Preferably the tanned valves are incorporated into theprostheses within about two days of their procurement.

FIG. 4 shows an aortic porcine valve, generally designated 11, that wastaken from a pig approximately 6 months old, tanned pursuant to theabove procedure, and trimmed for inclusion into a heart valveprosthesis, generally designated 12 in FIGS. 1, 2 and 8. The interior ofvalve 11 is comprised of three cusps or leaflets 13, 14, 15 whose basesare integral with the muscle tissue 16 that defines the aortic orificeand whose apices extend into the orifice to meet at axis 17 generallycentrally (see FIG. 3) of the orifice. Cusps 13, 14, 15 are joined atcommissures 18, 19, 22.

Valve 11 is supported in prosthesis 12 by a stent frame generallydesignated 23 and shown separately in FIGS. 3 through 6. As seen inFIGS. 3 and 4, frame 23 has an irregular ring shape and is comprised ofthree spaced, curved, ventricular struts or posts 24, 25, 26interconnected by three annular, ventricularly relieved sections 27, 28,29. The curvature of the ventricular edge 32 of frame 23 generallyfollows the bases of valve cusps 13, 14, 15. The atrial edge 33 of frame23 has atrial apices and depressions that generally register with therelieved sections and struts, respectively, of ventricular edge 32. Thespacing between struts 24, 25, 26 is unequal (detailed in FIG. 3) andstruts 24 and 25 are biased radially inwardly (FIGS. 3, 6B and 6D). Insmaller stents strut 26 tends to be generally parallel to axis 17 (FIGS.3 and 6F), whereas in larger stents it tends to be biased radiallyinwardly but to a much lesser degree than struts 24, 25. Correlativelyannular sections 27, 28 are biased radially outwardly FIGS. 6C and 6E),whereas annular section 29 is generally parallel to axis 17 (FIG. 6A) inthe case of smaller stents and in the case of larger stents tends to bebiased radially outwardly but to a much lesser degree than sections 27,28. As shown in FIGS. 5 and 6, the respective heights of the framestruts and annular sections are unequal. The inner surface of the framedefined by annular sections 27, 28, 29 and the bases of struts 24, 25,26 is generally convex outwardly.

The above described configuration of frame 23 is anatomically identicalto valve 11. Specifically the interior configuration of frame 23 is thenegative of the exterior configuration of valve 11. In this manner frame23 provides a near perfect fit and support for valve 11. Theconfigurations of frames made from valves taken from pigs of differentages than the pig from which valve 11 was taken will be essentiallyidentical to the configuration of frame 23; but their sizes will becorrespondingly smaller or larger than frame 23, as the case may be.

The anatomical configuration of frame 23 is achieved by molding it froma tanned, expanded valve, such as valve 11, as follows. A rubber castingof the interior of the valve is made. This casting serves as a supportfor the valve. The valve is then placed over the rubber casting and thevalve/casting assembly is dipped into a curable liquid rubber, such asthe silicone rubber sold under the trade designation Silastic RTV, toform an exterior coating on the assembly. After the coating has set theassembly is separated from the coating. The coating thus forms a mold ofthe exterior of the valve. From this rubber mold a rubber casting of theexterior of the valve is made. From this casting of the valve exterior awax casting is made. Finally a metal mold of the exterior of the valveis made from the wax casting by the lost wax technique. Plastic stentframes, such as frame 23, may be made from the metal mold by injectionmolding.

The material from which frame 23 is made and the thickness of frame 23may be such that frame 23 is bendable and resilient so that the struts24, 25, 26 are able to bend radially to accommodate the in vivo movementof the valve cusps. Thermoplastic materials to which the body has littleor no reactivity, such as Delrin polymer (a polyformaldehyde of greaterthan 15,000 molecular weight sold by DuPont), Lexan polymer (apolycarbonate), Nylon polymer (a hexamethylene diamine-adipic acidpolymer), high density polyethylene and polypropylene, may be used tomake the frames. The thickness of frames made of such materials willdepend on the size of the valve for which the frame is made. Largerframes are used for mitral valve replacements and should be thicker thanthe samller frames used for aortic valve replacements. Usually thethickness will be in the range of about 0.5 to about 1.5 mm, withthicknesses in the lower end of this range being used for the smallerframes and those in the higher end for the larger frames. Frames made ofDelrin polymer are preferred.

The exterior of frame 23 is covered by a nonabsorbent fabric cover 34(FIGS. 4 and 7). Cover 34 inhibits thrombus formation, promotes tissuegrowth and endothelialization of exposed prosthetic surfaces andprovides means for receiving the sutures by which valve 11 is emplacedwithin the stent frame. Fabric cover 23 is desirably multilayered andmay be formed from available surgical woven, mesh or sponge materials.Cover 34 has a thickened portion or bead 35 around its entire perimeterwhich may be formed either by rolling the edge of the cover or by sewingor otherwise affixing additional material thereto. The cover 34 shown inthe drawings has two layers and bead 35 was formed by rolling the edgeof the layers. The material from which cover 34 is formed should beunreactive to body tissue and fluids. It is desirable to usethermoplastic materials which have a substantially higher melting pointthan the material from which frame 23 is made, since such materials maybe readily and effectively attached to frame 23 by heat lamination.

The cover 34 shown in the drawings was attached to frame 23 as follows.Open mesh Dacron polymer (polyethylene terephthalate) cloth (No. 6050,USCI Co) was doubled and a ring 36 (FIGS. 4 and 7) was sewn into apocket 37 formed in the cloth. Ring 36 provides a site for suturingprosthesis 12 into the heart opening. It may be made from appropriatemedical grade, synthetic cloths, meshes, or felts. Dacron polymer orTeflon polymer (polytetrafluoroethylene) felt has been found to beespecially useful. The doubled cloth is then positioned over theexterior of frame 23 and a heating element, eg, a soldering iron, is runslowly around the exterior perimeter of frame 23. The temperature of theheating element should be above the melt temperature of frame 23 andbelow the melt temperature of cover 34. This causes the material offrame 23 to melt at said perimeter and the cover to be imbedded in themolten frame material. Once the heating element is removed the moltenframe material solidifies quickly thereby bonding the cover 34 to theexterior perimeter of the frame 23. The edge of the cover 34 outwardlyof the bond is trimmed, rolled twice and sewn down with a thread 38 toform bead 35.

Valve 11 is assembled within frame 23 as follows. Valve 11 is firstseated within frame 23 with commissures 18, 19, 22 generally lined upwith frame struts 25, 26, 24 respectively. Because the interiorconfiguration of frame 23 is substantially the negative of the exteriorconfiguration of valve 11 the latter sits snugly within the former. Theventricular edge of valve 11 is then sutured to the bead 35 along theventricular edge 32 of frame 23 (FIG. 2) and the atrial edge of valve 11is similarly sutured to the bead 35 along the atrial edge 33 of frame 23(FIG. 8) with sutures 42. Any excess muscle tissue 16 may be trimmedaway during the suturing. This completes the assembly of prosthesis 12and it is ready for implantation within a patient at any intracardiacposition. Implantation procedures are well-established in the surgicalart and do not require elaboration herein.

FIGS. 9-12 illustrate another heart valve stent, generally designated43, of this invention. Stent 43 is essentially identical to the stent ofFIGS. 1-8 except as regards its cloth covering 44 and the size of itsframe, generally designated 45.

As seen in FIG. 11 cloth covering 44 covers both the interior andexterior surfaces of frame 45. It comprises a first layer 46 that coversthe interior and exterior surfaces of frame 45 and a second layer 47that covers only the exterior surface of frame 45 and is locatedinwardly of layer 46. Layers 46, 47 are bunched to form an annularU-shaped thickened portion 48 that extends completely around frame 45.The ends 49, 52 of portion 48 are tied together by a thread 53 and thebase of portion 48 is tied by another thread 54. Portion 48 serves thesame purpose as ring 36 of the stent of FIGS. 1-8. Layers 46, 47 arefolded and tied together by a thread 56 along the ventricular edge offrame 45 to form a ventricular bead 56 and are similarly tied togetheralong the atrial edge of frame 45 by another thread 57 to form an atrialbead 58. Beads 56, 58 serve the same purpose as bead 35 of the stent ofFIGS. 1-8. Unlike the cloth cover 34 of the stent of FIGS. 1-8, clothcover 44 is not bonded directly to frame 45.

Referring to FIGS. 10 and 12 frame 44 has an irregular ring shape and iscomprised of three spaced struts 61, 62, 63 that are interconnected bythree sections 64, 65, 66. These struts and interconnecting sectionshave generally the same spacial attitudes and serve the same purposes asthe corresponding struts and interconnecting sections of the frame ofthe stent of FIGS. 1-8. However, the transverse dimensions of the formerare generally smaller than those of the latter. Such smaller dimensionsare desirable because they make the stent lighter and less bulky. Ofcourse the transverse dimensions of the struts and interconnectingsections should not be so small as to adversely affect the structuralintegrity or operation of the stent. In this regard the preferredtransverse dimensions (measured at the mid-point) of the struts is 3 to4 mm. The preferred transverse dimension (measured at its widest point)of the right coronary interconnecting section (64 in FIGS. 10 and 12,and 27 in FIGS. 4-6) will vary with the overall size of the frame,getting progressively larger as the overall size of the frame increases.For instance, in a stent for a 20 mm valve it is preferably about 5 mm,for a 24 mm valve preferably about 6.4 mm, for a 28 mm valve about 7.4mm and so forth. Thus the transverse dimension of the right coronaryinterconnecting section will normally be in the range of 5 to 8 mm,depending on the overall frame size. The transverse dimension of each ofthe other two interconnecting sections (65, 66 in FIGS. 10-12 and 28, 29in FIGS. 4-6) is preferably about 2 to 3 mm at its widest point.

The components of stent 43 may be made from the same materials as thecomponents of the stent of FIGS. 1-8. Likewise frame 45 may be made inthe same manner as frame 23 of the stent of FIGS. 1-8 so that it too isanatomically configured, and a valve, such as valve 11, may be assembledwithin stent 43 in the same manner that valve 11 is assembled withinframe 23.

Modifications of the stents, prostheses and procedures for making thesame described above which are obvious to those skilled in theprosthesis, surgical and polymer arts are intended to be within thescope of the following claims. Such modifications include, withoutlimitation, the use of other materials not specifically mentioned abovefor the stent frame and/or fabric cover, varying the size and locationof ring 36 or portion 48 to adapt the prosthesis for use as a mitral ortricuspid valve replacement, as the case may be, and, in the case of anexterior cover only, affixing the cover to the frame edge by means otherthan heat lamination, such as with adhesives.

We claim:
 1. A stent for a tanned natural tissue heart valve prosthesiscomprising:a. a plastic frame the interior configuration of which issubstantially the negative of the configuration of the exterior of saidvalve; and b. a fabric cover covering at least the exterior surface ofthe frame.
 2. The stent of claim 1 including:c. means along the frameperimeter providing a site for attaching the valve to the stent.
 3. Thestent of claim 2 wherein said means is a thickened portion of the cover.4. The stent of claim 2 wherein said means comprises a rolled edge ofthe cover.
 5. The stent of claim 1 wherein the cover includes athickened annular exterior portion providing a site for attaching theprosthesis intracardially.
 6. The stent of claim 1 wherein the frame hasan irregular ring shape and comprises three unequally spaced ventricularstruts of unequal height, two of which are biased radially inwardly andthe third of which is biased radially inwardly but to a lesser degreethan the other two or is generally parallel to the frame axis and threeannular ventricular relieved portions integral with and respectivelyinterconnecting said struts, said portions being of unequal height, twoof which are biased radially outwardly and the third of which is biasedradially outwardly but to a lesser degree than the other two or isgenerally parallel to the frame axis, the inner surface of the framedefined by the inner surfaces of the bases of said struts and the innersurfaces of said portions being generally convex outwardly.
 7. The stentof claim 1 wherein the portion of said cover covering said exteriorsurface is multilayered.
 8. The stent of claim 1 wherein the covercovers only the exterior surface of the frame and is affixed to theframe along the exterior perimeter thereof.
 9. The stent of claim 8wherein the material forming said cover is a thermoplastic having asubstantially higher melt temperature than said frame and said cover isaffixed to the frame by heat lamination.
 10. The stent of claim 1wherein the cover covers the entire surface of the frame.
 11. The stentof claim 1 wherein the thickness of said frame is about 0.5 to about 1.5mm.
 12. The stent of claim 1 wherein said frame is formed of Delrinpolymer and said cover is formed of Dacron polymer.
 13. The stent ofclaim 1 including:c. means along the frame perimeter providing a sitefor attaching the valve to the stent and wherein the cover covers theentire surface of the frame and includes a thickened annular exteriorportion providing a site for attaching the prosthesis intracardially,the thickness of the frame is about 0.5 to about 1.5 mm and the framehas an irregular ring shape and comprises three unequally spacedventricular struts of unequal height having a transverse midpoint widthof 3 to 4 mm, two of which are biased radially inwardly and the third ofwhich is biased radially inwardly but to a lesser degree than the othertwo or is generally parallel to the frame axis, and three annularventricular relieved portions integral with and respectivelyinterconnecting said struts, said portions being of unequal height withthe right coronary portion having a transverse dimension at its widestpoint of 5 to 8 mm and the other two portions having a transversedimension at their widest points of 2 to 3 mm, two of the portions beingbiased radially outwardly and the third being biased radially outwardlybut to a lesser degree than the other two or being generally parallel tothe frame axis, the inner surface of the frame defined by the innersurfaces of the bases of said struts and the inner surfaces of saidportions being generally convex outwardly.
 14. A heart valve prosthesiscomprising:a. the stent of claim 1; and b. a tanned natural tissue heartvalve affixed within said stent.
 15. The heart valve prosthesis of claim14 wherein the valve was tanned with glutaraldehyde in an expandedstate.
 16. The heart valve prosthesis of claim 14 wherein the valve wastanned with a 0.5% to 10% glutaraldehyde solution under a pressure ofabout 20 to about 100 mm Hg.
 17. The heart valve prosthesis of claim 14wherein the valve was tanned with a 5% glutaraldehyde solution under apressure of about 50 mm Hg.
 18. The heart valve prosthesis of claim 14wherein the valve is a porcine aortic valve.
 19. A heart valveprosthesis comprising:a. the stent of claim 2; and b. a tanned naturaltissue heart valve affixed within said stent.
 20. A heart valveprosthesis comprising:a. the stent of claim 6; and b. a tanned naturaltissue heart valve affixed within said stent.
 21. A heart valveprosthesis comprising:a. the stent of claim 10; and b. a tanned naturaltissue heart valve affixed within said stent.
 22. A heart valveprosthesis comprising:a. the stent of claim 13; and b. a tanned naturaltissue heart valve affixed within said stent.
 23. The heart valveprosthesis of claim 22 wherein the valve is a porcine aortic valvetanned with glutaraldehyde in an expanded state.
 24. The heart valveprosthesis of claim 23 wherein the valve is a porcine aortic valvetanned with a 0.5% to 10% glutaraldehyde solution under a pressure ofabout 20 to about 100 mm Hg.